How do large snowflakes form?

Posted on March 5, 2012 by Weather Guys Editor

There are four basic shapes of ice crystals: the hexagonal plate, the needle, the column and the dendrite. The dendrites are hexagonal with elongated branches, or fingers, of ice; they most closely resemble what we think of as snowflakes. The temperature at which the crystal grows determines the particular shape. A snowflake is an individual ice crystal or an aggregate of ice crystals. Large snowflakes are aggregates of ice crystals.

Aggregation is the process by which ice crystals collide and form a single larger ice particle. The probability that two crystals will stick together depends on the shape of the crystals. If two dendrites collide, it is likely that their branches will become entangled and the two crystals will stick together. When two plates collide there is a good chance that they will simply bounce off one another.

Temperature also plays a role in aggregation. If the temperature of one crystal is slightly above freezing, it may be encased in a thin film of liquid water. If this particle collides with another crystal, the thin film of water may freeze at the point of contact and bond the two particles into one. (Similar physics underlies why you should never lick a cold metal flag pole.)

Snowflakes composed of aggregates can sometimes reach 3 or 4 inches in size. The record-size snowflake occurred in January 1887 in Fort Keough, Mont., when some flakes were measured at 15 inches in diameter. That is about the size of a family-sized pizza pie.

Category: Phenomena
Tags , ,
Comments Off on How do large snowflakes form?

Does the warm winter mean a warm spring and summer?

Posted on February 27, 2012 by Weather Guys Editor

We have continued to enjoy temperatures well above normal through most of February 2012, making this year’s Dec. 1–Feb. 20 the fifth-warmest on record with an average temperature of 28.3F in Madison.  Barring an exceptionally warm last week of the month (which does not appear likely), that is where we will end up — the fifth-warmest winter (defined as December, January, February) of all time in Madison.

A natural next question is what does the winter warmth portend for the coming spring and summer.  Colleagues at the National Weather Service Forecast Office in Sullivan, Wis.,  have thought about this question.  Though speculation about such a connection is derived purely from statistics, the statistics do tell an interesting story.

First, let’s define spring as March, April and May and summer as June, July and August.  Then, ranking the past 141 years of springs and summers from warmest (rank 1) to coldest (rank 141), they have calculated the average rank for the springs and summers following the 20 warmest winters in Milwaukee (results likely very similar for Madison).

It turns out that the spring following an exceptionally warm winter, on average, has been warmer than 60 percent of all springs.  Even more intriguing, the summers following exceptionally warm winters have been warmer, on average, than 69 percent of all summers.  Those are pretty good odds for a warmer-than-normal spring and summer.

Category: Seasons
Tags , , , ,
Comments Off on Does the warm winter mean a warm spring and summer?

Who first suggested human activities could change our climate?

Posted on February 20, 2012 by Weather Guys Editor

Svante Arrhenius, a Swedish scientist, is probably the first scientist to propose that burning of fossil fuels could modify our global temperatures. In 1896, Arrhenius recognized that carbon dioxide, a byproduct of burning carbon substances such as natural gas, gasoline and oil, would act like a greenhouse gas.

A greenhouse gas traps heat by allowing energy from the sun to pass through the atmosphere and heat the surface, and then the atmosphere absorbs heat emitted by the ground. Carbon dioxide, water vapor and methane gases are examples of greenhouse gases.

Greenhouse gases occur naturally in the atmosphere. If we did not have greenhouse gases, Earth’s average temperature would be about 60 degrees colder than it is now.

Too much greenhouse gases, on the other hand, could lead to a hotter planet. Venus is an example of how large concentrations of greenhouse gases — carbon dioxide in its case — can lead to a very hot planet. The large concentrations of carbon dioxide on Venus result in an average surface temperature of about 600 degrees.

Arrhenius, a chemist, postulated that adding carbon dioxide through human activities could lead to a warmer Earth. He estimated that global temperatures could rise as much as 10 degrees.

Today, climate scientists estimate that by the end of the 21st century, increased human activities could result in a global warming of 3 to 7 degrees relative to the period between 1980-1990. Land areas will warm more than oceans, and winters will likely warm more than summers.

Category: Climate
Tags
Comments Off on Who first suggested human activities could change our climate?

Just how warm has this winter been?

Posted on February 13, 2012 by Weather Guys Editor

This winter began much warmer than normal, but where does this year’s mildness rank all-time? You might be surprised to learn that although we are in the top 10 warmest starts to winter, we are not really close to the top. In fact, we are experiencing only the sixth warmest Dec. 1-Feb. 8 in Madison history with an average temperature of 28.3 degrees Fahrenheit during that stretch.

Only one other year within the last 10 makes the same top 10 list, and that was 2001-2002, which came in at No. 3 with an average Dec. 1- Feb. 8 temperature of 28.9 F.

The all-time warmest such period occurred in 1877-1878, when the average temperature was 31.9 F — incredibly warm. That winter went on to be the warmest (December, January, February) ever in Madison with a daily average temperature of 32 F. In fact, of the 12 winters that made the top 10 list for Dec. 1 — Feb. 8 (there were three tied at 10th place), nine of them also made the top 10 list for warmest winter in Madison.

Very warm Februarys in 1931, 1954, and 1998 (when Lake Mendota was ice free on Feb. 28, the only February end to the ice season on record) put those winters in the top 10 for warmest winter even though they did not get off to as warm a start as this year has.

So, it seems likely that our winter will end up being one of the warmest but not even close to the warmest.

Category: Seasons
Tags , , ,
Comments Off on Just how warm has this winter been?

What is the concept of heating degree-days?

Posted on February 6, 2012 by Weather Guys Editor

Each degree that the mean temperature is below 65 degrees is one heating degree-day. So, if today’s average temperature is 55 degrees, that is 10 heating degree-days.

Engineers determined that when the mean outdoor temperature drops below 65 degrees, most buildings require heating to maintain an indoor temperature of 70 degrees. In an effort to make weather data easy to use in planning, they developed this definition to estimate fuel consumption needs. Fuel distributors often use this index to schedule home deliveries. Since energy consumption in homes derives primarily from natural gas and electricity, these utilities predict power demands by adding up this index over time.

The amount of heat required to maintain a building’s temperature is proportional to the accumulated heating degree-days. So, heating degree-day totals are usually reported each day, as well as a total for the season. This allows us to quickly judge whether the season is above, below or near normal.

Over a year, northern Wisconsin has about 9,000 heating degree-days, while southern Wisconsin has about 7,000 heating degree-days. Over a year the indoor heating requirements for southern Wisconsin are about 7/9ths that of northern Wisconsin.

This past January most of southern Wisconsin had temperatures that were 4 degrees above normal while December’s temperatures were more than 5 degrees above the mean. With those warm temperatures, Madison’s cumulative heating degree-days this winter is a few hundred below normal.

Category: Climate
Tags
Comments Off on What is the concept of heating degree-days?